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Nitrogen runoff dominates water nitrogen pollution from rice-wheat rotation in the Taihu Lake region of China
Agriculture, Ecosystems and Environment 156 (2012) 1–11
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Nitrogen runoff dominates water nitrogen pollution from rice-wheat rotation in the Taihu Lake region of China Xu Zhao a , Yang Zhou b , Ju Min a , Shenqiang Wang a,∗ , Weiming Shi a , Guangxi Xing a,∗ a State Key Laboratory of Soil and Sustainable Agriculture, Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China b Resource and Environmental College, Anhui Agricultural University, Hefei 230036, China
a r t i c l e
i n f o
Article history: Received 8 December 2011 Received in revised form 23 April 2012 Accepted 25 April 2012 Keywords: Rice-wheat rotation Runoff Leaching Water pollution Conventional management
a b s t r a c t Nitrogen (N) runoff and leaching are two direct pathways of N pollution from agricultural land to water systems. It is not yet conclusive whether intensive rice-wheat rotation system contributes to N pollution in the surrounding water systems in the Taihu Lake region (TLR). We conducted a three-year field experiments to monitor N runoff and leaching from rice-wheat paddy soil. Annual N runoff and leaching was in the range of 55.3–93.1 kg N ha−1 , with a mean of ca. 69.2 kg N ha−1 . Of the total N export to water systems, 82–93% was from N runoff and only 7–18% was from N leaching. For seasonal distribution, the wheat season contributed 57–85% of total N, while the rice season contributed 15–43%. Because of different water practices, N runoff and leaching exhibited completely different patterns in two crop seasons. Considerable N runoff in wheat season can mainly be attributed to moderately heavy rainfall and accelerated water flow in existing drainage ditches. Large variations in N runoff during rice season were predominantly due to variable natural precipitation and irregular artificial draining. N leaching appeared to be related to fertilization and precipitation in wheat season, while it was associated with fertilization and soil depth during rice season. These field data suggest that runoff dominated N export from rice-wheat rotation under conventional N and water management contributes potentially to water pollution in the TLR. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The Taihu Lake region (TLR) in the Yangtze River delta (Fig. 1) covers 36,500 km2 . As one of the five major rice production regions in China, TLR has 75% of its total land growing rice. Rice-wheat rotation has been practiced here for thousands of years (Li, 1992). This region is one of the most fertilized regions with nitrogen (N) in the world. Currently, the annual application rate of synthetic N fertilizers is as high as 500–600 kg N ha−1 for two crops (Xing et al., 2002). There is, therefore, growing concern about environmental impacts induced by the high N loads (Zhu et al., 2006). N runoff and leaching from rice-wheat paddy fields was not well documented until the late 1990s (Gao et al., 2004; Guo et al., 2004a,b; Ju et al., 2009; Liang et al., 2010; Tian et al., 2007; Zhao et al., 2009a; Zhu and Chen, 2002; Zhu et al., 2000). Under current standard farming conditions in the TLR, the rice and
∗ Corresponding authors. Tel.: +86 25 86881534; fax: +86 25 86881028. E-mail addresses: [email protected] (X. Zhao), [email protected] (S. Wang), [email protected] (G. Xing). 0167-8809/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agee.2012.04.024
wheat seasons follow a distinct water regime. Flooding the field alternating with frequent draining (e.g., for midseason aerations and harvest) is common practice during rice season. Contrarily, rainwater is the only source of soil moisture during wheat season (Zhao et al., 2011). Despite of less rainfall in wheat season than in rice season, it is still excessive for normal growth of wheat plants due to subtropical monsoon climate; drainage ditches are usually needed to protect the wheat plants from waterlogging injury. These distinct water schemes can influence the transformation and migration of N, and consequently cause great variations in runoff and leaching N losses between rice and wheat seasons. Our previous lysimeter study reported that N runoff and leaching are greater in wheat season than in rice season (Zhao et al., 2009a), although elaborate explanations for this phenomenon are not available. The variation of N runoff and leaching patterns in both crop seasons is also not well characterized. So far, it is still unclear whether intensive rice production is one of major contributors to water N pollution in the TLR. Because of the existence of field levees and plough pans with low permeability, N runoff and leaching from paddy soil are considered to be minimal (Zhu et al., 1997). Their contribution to water pollution is often insignificant in comparison with that from the increasing amount
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Fig. 1. Location of the Taihu Lake region (TLR) in China. The star denotes the monitoring site.
of human and animal excreta (Xing et al., 2001) and atmospheric N deposition (Xie et al., 2007). However, a number of studies have concluded that the large inputs of synthetic N fertilizer used in rice production are one of the major sources of N pollution in water bodies (Gao et al., 2004; Guo et al., 2004a,b; Liang et al., 2010; Tian et al., 2007; Zhu et al., 2000, 2003). One likely explanation for the divergent results is that appropriate methods for in situ observation of N runoff and leaching are lacking. Although N concentrations in runoff and leaching water can be accurately determined, the amount of water flow is not actually measured in paddy field. It is usually large-scale estimates by water balance methods using multifarious parameters from soil hydroconductibility coefficients, soil evaporation, crop transpiration, irrigation, precipitation, etc. (Allen et al., 1998; Tian et al., 2007; Zhu et al., 2000), which may not reflect the true conditions of the micro-environment. Consequently, an amount of uncertainty is introduced into the estimates of N exports via runoff and leaching. In addition, previous studies have been mostly based on short-term observations of either a crop rotation or a single N pathway (i.e., either runoff or leaching) from either small plots or monolith lysimeters. Lack of consecutive year-round investigations on both N runoff and leaching one time at field scale might be another alternative explanation for the current divergent results of N direct export to water systems. Since June 2007, we had conducted a three-year-round consecutive field-scale observation to measure total N input and N output of rice-wheat rotation agroecosystem with improved metering methods for water flow, including irrigation, runoff and leaching. The entire N balance and its contributing input/output factors in this high-N-input agricultural system were discussed separately (Zhao et al., 2012). The emphasis of this paper therefore was on variation patterns in N runoff and leaching and their key factors. We aim to examine how much the potential contribution of direct N export is from intensive rice-wheat rotation system to water N pollution, and illustrate how the conventional water management affects N export via water flow in rice and wheat seasons. The information could reveal insights for reducing non-point pollution of water from paddy field in TLR.
2. Materials and methods 2.1. Study area and rice field plot experiment The experiment was located in the midwest of the TLR, on the northwest side of Taihu Lake, within 1 km of the shore (Fig. 1). This location has a subtropical monsoon climate with an average temperature of 15.7 ◦ C and an annual rainfall of 1177 mm. The paddy soil is classified as Gleyi–Stagnic Anthrosols, and originated from a Lacustrine deposit as parent material. The soil consists of 8.30% sand, 81.5% silt, and 10.2% clay by volume, and contains 15.4 g kg−1 organic C, 1.79 g kg−1 N, and 11.8 cmol kg−1 CEC. The pH (H2 O) of the soil (0–15 cm) was 5.6. A piece of paddy field (Fig. 2) belonging to a local farmer was selected for observation. It is surrounded by large paddy fields cultivated by other local farmers. The paddy field was treated with urea at rates of 300 kg N ha−1 for rice season and 200 kg N ha−1 for wheat season. For N application, 30% was basally applied, 40% was top-dressed at the tillering stage, and the remaining 30% was topdressed at the ear differentiation stage for each crop. Phosphate and potassium fertilizers were applied basally in the form of superphosphate at a rate of 60 kg P2 O5 ha−1 and in the form of potassium chloride at a rate of 45 kg K2 O ha−1 . In this area, the following normal cultivation and field management practices in the TLR are usually adopted in the ricewheat rotated paddy fields. Firstly, approximately 240–300 and 200–250 kg N ha−1 of chemical N fertilizers are routinely applied to the soil to grow rice in summer and wheat in winter, respectively. Secondly, direct sowing and surface fertilizer application are employed due to convenience and economic costs in practice. Thirdly, the current standard water regime is popularly practiced. During rice seasons, flooded water was mostly maintained at a depth of 3–5 cm in the field except for drainage before sowing, several midseason aerations and final drainage events. At the beginning of each wheat season, drainage ditches (15 cm in depth and 10 cm in width; were usually spaced 1.5–2 m apart) were opened mechanically in the field (Fig. 2). In addition, a local crop grower controlled above-mentioned normal cultivation and field
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
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Fig. 2. Plane figure of the paddy field and landscape of drainage ditches during the period of wheat growth. Those drainage ditches were usually 15 cm in depth and 10 cm in width with 1.5–2 m apart from each other. This conventional practice can effectively prevent waterlogging that might injury wheat plants due to frequent rain events during the period of wheat growth.
management practices in the experimental field (Table 1). Thus, the results of the study were expected to be appropriately representative of the paddy fields under conventional farming in the TLR. 2.2. Sampling and analysis Porous pipes made of polyvinyl chloride were installed in triplicate at different soil depths to collect percolation water (Fig. 3a). Table 1 Dates of cultivation and management during the three rice-wheat rotations from 2007 to 2010. Crop rotation
Activity
2007 rice
Preflooding, June 6, 2007; Basal fertilization, June 8; Draining and sowing, June 9; Re-flooding, June 19; First top-dressing, June 25; Mid-season aeration, July 18–28; Second-top dressing, July 30; Mid-season aeration, August 4–8; Mid-season aeration, August 10–21; Final drainage, October 9; Harvesting, October 28. Sowing, November 14, 2007; basal fertilization, November 14; First top-dressing, December 15; Second top-dressing, February 7, 2008; Harvesting, May 24. Pre-flooding, June 11, 2008; Draining and sowing, June 14; Re-flooding, July 1; Basal fertilization, July 7; First top-dressing, July 21; Mid-season aeration, July 27 to August 3; Second-top dressing, August 6; Mid-season aeration, August 16 to August 23; Final drainage, October 3; Harvesting, October 18 Sowing, November 19, 2008; Basal fertilization, November 23; First top-dressing, December 26; Second top-dressing, February 10, 2009; Harvesting, May 24. Pre-flooding, June 4, 2009; Draining and sowing, June 7; Re-flooding, June 14; Basal fertilization, June 18; First top-dressing, July 5; Mid-season aeration, July 18–20; Mid-season aeration, July 26 to August 3; Second-top dressing, August 4; Mid-season aeration, August 14–23; Mid-season aeration, August 27 to September 3; Final drainage, October 14; Harvesting, October 23. Sowing, November 5, 2009; Basal fertilization, November 8; First top-dressing, December 4; Second top-dressing, February 24, 2010; Harvesting, June 3.
2007/2008 wheat
2008 rice
2008/2009wheat
2009 rice
2009/2010 wheat
Percolation water was collected with a vacuum hand pump usually at 10-day intervals during rice season, but more frequently immediately after fertilizer application. Likewise, percolation water was collected after every rain event during wheat season. Water preexisting in the porous pipe was drained and discarded before sampling to avoid contamination. Runoff water in both seasons was sampled at drainage outlet point when every drainage event occurred. Samples of irrigation water in rice season were taken for analysis at irrigation inlet point (Fig. 2) when every irrigation event occurred. Percolation, runoff and irrigation water samples were stored in 250 ml plastic bottles, and immediately frozen without filtering at −20 ◦ C in a freezer until analysis. Concentrations of NH4 + , NO3 − , and total N (TN) in these water samples were analyzed with a continuous-flow N analyzer (Skalar, Netherlands, with analytic error ± 3.9% and low detection limit of 0.2 mg N L−1 ). Three improvements in methodology varying from indirect estimates using water balance method were explored in the current study to directly obtain the amount of water export: (1) two electromagnetic flowmeters (Fig. 2) were installed to precisely measure the water volume of individual irrigation and runoff events on the field scale; (2) a slightly modified rapid-response percolation meter (International Rice Research Institute, 1987; Fig. 3b) was designed to actually observe the volume of water transferred via leaching during rice season; (3) field undisturbed tension-free monolith lysimeters with a special design (Fig. 3c; Wang et al., 2011) were used to proximately obtain the volume of water transferred via leaching in wheat season. Cumulative N runoff = sum of amount of individual runoff × (m3 ha−1 ) × N concentration in runoff water (mg N L−1 ) × 0.001
Cumulative N leaching (kg N ha−1 ) = time-interval weighted N concentration at 100 cm soil depth (mg N L−1 ) × total amount of leaching water (m3 ha−1 ) × 0.001 where time-interval weighted N concentration is defined as sum [(individual N concentration (mg N L−1 ) × intervals between two adjacent samplings (days)]/the total growth time (days).
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Fig. 3. Schematics view of porous pipes (a), rapid-response percolation meter (b) and the field undisturbed tension-free monolith lysimeter (c). The percolation meter was a modification type based on the meter designed by International Rice Research Institute (1987).
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
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Table 2 Average N concentration in percolation water from different soil depths and estimation of N leaching in the three rice-wheat rotations from 2007 to 2010. Crop season
Soil depth (cm)
Average N concentration in percolation water (mg N L−1 )a NO3 −
NH4 +
Growth period (day)
Leaching waterb (m3 ha−1 )
Total N
N loads in leaching water (kg N ha−1 )c NO3 −
NH4 +
Total N
2007 rice
20–40 40–60 60–80 80–100
1.27 0.94 0.52 0.41
± ± ± ±
0.21 0.31 0.32 0.25
2.44 1.65 0.77 0.84
± ± ± ±
1.34 0.6 0.53 0.74
4.28 3.08 1.71 1.80
± ± ± ±
1.54 1.05 1.06 1.28
144
1920
0.79 ± 0.47
1.61 ± 1.43
3.46 ± 2.46
2008 rice
20–40 40–60 60–80 80–100
3.15 2.65 2.34 0.78
± ± ± ±
0.25 0.92 0.92 0.35
1.51 1.28 1.83 1.63
± ± ± ±
0.87 0.80 1.72 1.10
5.03 4.53 4.56 3.00
± ± ± ±
0.83 0.55 1.79 1.86
129
1940
1.51 ± 0.68
3.15 ± 2.14
5.82 ± 3.60
2009 rice
20–40 40–60 60–80 80–100
0.29 0.38 0.34 0.37
± ± ± ±
0.06 0.18 0.23 0.14
0.93 0.28 0.35 0.77
± ± ± ±
0.52 0.10 0.21 0.25
2.28 1.13 1.15 2.20
± ± ± ±
0.60 0.33 0.26 0.14
141
2180
0.81 ± 0.55
1.68 ± 0.55
4.79 ± 0.30
2007/2008 wheat
20–40 40–60 60–80 80–100
7.77 5.29 8.52 1.71
± ± ± ±
0.49 1.89 3.61 1.37
0.14 0.25 0.26 0.11
± ± ± ±
0.03 0.09 0.10 0.03
10.4 5.69 8.97 3.81
± ± ± ±
1.93 2.08 3.57 2.91
192
138
0.24 ± 0.19
0.02 ± 0.00
0.53 ± 0.40
2008/2009 wheat
20–40 40–60 60–80 80–100
15.7 12.6 7.47 14.9
± ± ± ±
11.0 3.48 3.29 8.63
0.28 0.29 0.20 0.31
± ± ± ±
0.12 0.10 0.02 0.32
20.7 15.6 8.37 17.5
± ± ± ±
16.0 4.97 3.25 9.50
186
230
3.42 ± 1.98
0.07 ± 0.07
4.02 ± 2.18
20–40 40–60 60–80 80–100
13.7 11.3 8.52 7.40
± ± ± ±
3.83 4.12 1.03 1.02
0.40 0.52 0.56 0.34
± ± ± ±
0.21 0.40 0.59 0.26
16.1 14.1 10.0 8.83
± ± ± ±
5.03 5.77 2.25 0.73
210
1180
8.73 ± 1.20
0.40 ± 0.31
2009/2010 wheat
10.4 ± 0.87
Date are time-interval weighted means ± SD of the three replicates from field measurements. b Amount of leaching water in rice season was estimated based on the vertical leaching rate of 2 mm day−1 during the flooded period; amount of leaching water in wheat season was obtained from the field undisturbed tension-free monolith lysimeter (with 38 cm in diameter,100 cm in height) which has the same conditions as field treatment. c Calculated as N concentration in percolation water from 100 cm soil depth multiplied by amount of leaching water. a
The total amount of leaching water in rice season is defined as the rate of surface water vertical percolation (mm day−1 ) × flooded periods (day) × 10. Flooding condition in most period of rice season provided an approximately constant rate of water moving through soil profile. Therefore, the rate of surface water vertical percolation in rice season during flooded periods was measured using the percolation meter as illustrated in Fig. 3b. Compared with the water balance method, this method significantly simplified the calculation of the leaching water amount and reduced the error. The amount of water that percolated vertically through the cylinder equaled the amount that passed through the transparent tube on the float. In the current study field, the rate of vertical percolation was averaged at 2 mm day−1 from 15 replicate measurements distributed well in the plot. Unlike rice season, rainfall is the only driver of water leaching in wheat season. Water leaching occurred randomly because the rainfall amount, intensity and frequency varied greatly. This made the observation of amount of leaching water so difficult in the field scale. So we used field undisturbed tension-free monolith lysimeters (Fig. 3c) to directly obtain the total amount of leaching water in wheat season. Three lysimeters were installed and cultivated exactly like the rice or wheat plants in the field experiment under the same water and fertilization management. A simulated underground water supply system could provide a 10 cm height water layer in the bottom of lysimeters and prevent drought cracking of the soil monolith caused by evaporation and transpiration. Two runoff outlets were also installed at 0 and 15 cm soil depths to simulate runoff on the soil surface and from the 15 cm deep drainage ditches. These special practices ensured that the amounts of leaching water obtained from the lysimeters were more realistically similar to those in the field. Although certain errors remain, the data obtained by lysimeters are at least measured values from
the field, varying from estimated values using the water balance method. One homemade quadrate rain sampler (0.49 m2 in area and 150 cm in height from the ground) was installed near the field to measure precipitation. Individual precipitation (mm) = rainwater collected in each precipitation event . 0.49 m2
3. Results and discussion 3.1. Characteristic variations of N leaching in relation to fertilization and soil depth during rice season For the three rice seasons, TN concentration in percolation water was always higher before July than after August (Fig. 4a). This implied that N leaching tended to occur in the early stages of rice growth, during which 70% of seasonal N fertilizer was applied as basal fertilizer and the first N top dressing (Table 1). However, the fluctuation intensity of TN concentration in percolation water declined with increased soil depth (Fig. 4a), suggesting that the effect of N fertilization on N leaching weakened with the depth of soil. Average TN concentration in percolation water from the top 40 cm of soil was far greater than that from deeper soil layers (Table 2). This indicated that a large amount of fertilizer N that leached from the cultivated layer (0–20 cm) was actually retained in the top 40 cm soil layer. For rice growing season, the rate of water moving through the 0–100 cm soil layer was approximately constant under flooding condition, so percentage distribution of percolated N in vertical soil profile can be calculated based on
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Fig. 4. Variation of N concentrations in percolation water from different soil depths in paddy field during the three rice (a) and wheat (b) seasons from 2007 to 2010. The arrow denotes the timing of N fertilization.
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11 Table 3 Vertical distribution of percolate N in 20–100 cm paddy soil profile during the three rice seasons from 2007 to 2010. Crop season
Soil deptha 20–40 cm
40–60 cm
60–80 cm
80–100 cm
2007 rice 2008 rice 2009 rice
39.4% 29.4% 33.7%
28.3% 26.5% 16.8%
15.8% 26.6% 17.0%
16. 6% 17.5% 32.5%
Average
34.2%
23.9%
19.8%
22.2%
a
For rice growing season, the rate of water moving through the 0–100 cm soil layer was approximately constant under flooding condition, so percentage distribution of percolated N in vertical soil profile can be calculated based on average concentrations of TN in percolation water from each soil depth.
average concentrations of TN in percolation water from each soil depth. The results (Table 3) showed that 58% of percolated N accumulated in the top 60 cm soil layer, and approximately 78% of percolated N was in the top 80 cm soil layer in rice season. Apparently, not all percolated N from the cultivated layer entered into phreatic water. This raises an interesting question: to what depth can percolated N be considered as leaching loss when N leaching in the paddy field is determined using suction cups or Teflon tubes? At present, the reported results on N leaching loss from paddy soil in the TLR differ substantially due to differences in methodology (e.g., backfill soil column, suction cups or Teflon tubes, in situ lysimeter), estimation method, the standard of soil depth regarded as leaching loss (40–100 cm), etc. Therefore, a standard observation and calculation method for N leaching is expected to be found on a regional scale for the comparison of results in rice paddies and evaluation of N leaching loss to groundwater. The estimates of N loads in percolation water from 100 cm were 3.46 to 5.82 kg N ha−1 (Table 2) during the three rice seasons. Based on the current investigation and previous studies (Tian et al., 2007; Zhao et al., 2009a; Zhu et al., 2000), it can be concluded that N leaching from most flooded rice paddies in TLR may not be as high as feared, because less than 6 kg N ha−1 was leached, and therefore the effect on N pollution of groundwater might not necessarily be higher than that of some other upland soil (Zhang et al., 2005). One important reason for the low rate of N leaching might be the puddled layer with low permeability at about 15–20 cm in depth. The percolated NO3 − was minimal during the three rice seasons, except in the short stage of wheat-rice alternation (Fig. 4a and Table 2); at that time, it might actually have originated from
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remnant NO3 − from the previous wheat season (Zhao et al., 2011). The low NO3 − concentration in percolation water was due to that flooding conditions during the rice seasons created an anaerobic soil environment for denitrification. The fact of excessive accumulation of NH4 + in percolated water indicated that after quick hydrolysis of applied urea with oxygen-limited nitrification under flooding conditions, NH4 + could still flow downward with infiltrated water (Fig. 4a). 3.2. Significant N leaching closely associated with precipitation and fertilization during wheat season Notable vertical movement of N existed in the soil profile during the three wheat seasons. This conclusion is supported by the following evidence: the 40 cm depth exhibited higher TN concentration values initially but lower values later than deeper soil depths at the later stages of wheat growth, and the high peaks of TN concentration were usually found in percolation water from 100 cm soil depth (Fig. 4b). The applications of fertilizer N did not always induce immediate peaks in N concentration in percolation water, implying that the majority of N that was not taken up by wheat plants might have accumulated in the cultivated soil layer after N fertilization. When a heavy rain event occurred, N could be flushed out and move downward through percolation and runoff. NO3 − leaching was the predominant form of N leaching during wheat season (Table 2 and Fig. 4b). The probable reason might be strong nitrification under aerobic soil conditions mainly created by the drainage ditches in wheat season. NO3 − originating from fertilization can accumulate in the cultivated soil layer and move downward through soil with water flow more easily than NH4 + due to its negative charge. Significant amounts of NH4 + could still migrate downward if intense storm events occurred immediately after the N application (as seen in the 2009/2010 wheat season; Fig. 4b). Rainfall was the significant factor in the leachate amount during wheat season. The precipitation amount, intensity, and frequency (Fig. 5) probably had important effects on the N concentration in percolation water (Fig. 4b) and might have caused great variations (0.53–10.4 kg N ha−1 ; Table 2) in the amounts of N leaching. Despite great variations in the amount of N leaching, the significant downward migration of NO3 − in the soil profile during wheat seasons suggests a risk of groundwater pollution in paddy soil. An investigation carried out in the 1990s in the TLR found that the concentration
Fig. 5. Precipitation and runoff water caused by precipitation during the three rice-wheat rotations from 2007 to 2010.
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Table 4 N runoff in paddy soils during the three rice-wheat rotations from 2007 to 2010. Rotation
Crop season
Total water runoff a
Precipitation
N runoff b Total N
(mm)
NO3 − (kg N ha−1 )
NH4 +
(m3 ha−1 ) 2007/2008
2007 rice 2007/2008 wheat
4701 1918
661 434
4.89 24.5
13.2 6.88
21.8 33.4
2008/2009
2008 rice 2008/2009 wheat
1535 1991
533 442
0.41 27.3
0.77 11.1
2.65 42.8
2009/2010
2009 rice 2009/2010 wheat
8824 3020
1055 563
9.60 32.9
3.48 19.7
19.2 58.7
a b
Total water runoff was the sum of individual runoff volume measured by an electromagnetic flowmeter at the drainage outlet. N runoff was the sum of amount of individual runoff (m3 ha−1 ) × N concentration in runoff water (mg N L−1 ) × 0.001.
of NO3 − averaged 7.89 mg N L−1 among 40 well water samples, and revealed that NO3 − in well water in the TLR was commonly associated with N leaching from farmland (Xing et al., 2001). 3.3. Variations of runoff N in rice season affected by precipitation and artificial draining Fig. 6a shows the great interseasonal variation in runoff N during each rice season. This variation was predominantly governed by the times of fertilization and runoff events. If runoff happened to occur soon after N fertilization, high N concentration was inevitably present in the runoff water. A similar phenomenon has been observed by other scientists (Kim et al., 2006; Tian et al., 2007; Yoshinaga et al., 2007). However, if runoff was delayed for one week or more after N fertilization, N concentration was low and stable in the runoff water (as seen in the 2008 rice season). The interseasonal variation in the amount of individual N runoff was the result of the influence of N concentration and the amount of runoff water. The observed runoff N from rice paddy soil varied from 2.65 to 21.8 kg N ha−1 during the three rice seasons (Table 4). This variation was largely dependent of the difference in volume and the frequency of water runoff, which was in connection with precipitation and artificial drainage. High amount of N runoff in 2007 and 2009 rice seasons could be attributed to their frequent runoff events caused by artificial draining and precipitation. Low amounts of N runoff during the 2008 rice season was due to fewer runoff events under the water regime of intermittent irrigation with drained soil by natural evaporation and percolation. Artificial draining is one important factor dominating N runoff in rice season. Currently, direct seeding instead of transplanting has become a popular cultivation pattern due to its convenience and cost economics in practice (Pandey et al., 2002). A drainage event is usually conducted after steeping the field (pre-flooding) in order to provide drained soil for rice seed broadcasting and the emergence of rice seedlings (which usually takes 10–15 days). Drainage events also occur during several midseason aerations (MSA) to inhibit ineffective tillering, and again at the mature stage before rice harvest. N runoff loss is inevitable due to these frequent artificial drainages.
Natural precipitation is another important cause of runoff during the flooded period of rice growth. In a flooded rice paddy, runoff is closely correlated with rainfall and the height of the field levees (Zhao et al., 2009a). For the study area, it is rare that rainfall would exceed 100 mm in rice season, so runoff directly through field levees seems impossible due to the height of the field levees (15–20 cm above soil surface). However, it is common practice to lower surface water in rainy days to prevent the inhibition effect of deep flooded water on root growth and plant tillering. The usual practice is to reduce the height of the drainage outlet to 5–7 cm, which may allow significant runoff if rainwater substantially exceeds the height of the drainage outlet due to a heavy rainfall event. We used an electromagnetic flowmeter (Fig. 2a) to precisely measure the amount of runoff water and firstly differentiate the contributions from different sources to N runoff during rice season. The average data from the three rice seasons showed that 59.1% of seasonal N runoff was caused by precipitation, 19.7% by drainage before sowing, and 21.2% by midseason aerations (Table 5). This was not consistent with the common knowledge that N runoff from paddy soil during rice season was minimal because of the existed field levees (Zhao et al., 2009a; Zhu et al., 1997). The reason was largely due to frequent artificial draining induced by diverse causes.
3.4. Considerable N runoff in wheat season attributed to existing drainage ditches and moderately heavy rainfall The variations in runoff amounts exhibited a pattern similar to the pattern of rainfall during the three wheat seasons (Fig. 5). Apparently, precipitation was the main factor governing the runoff. However, the runoff events were not completely synchronized with each rainfall event, but tended to happen immediately after successive rainfall events and heavy rainfall events (Fig. 5). 44–57% of the seasonal rainfall during the three wheat seasons was lost via drainage in the wheat fields (Table 4). This fact implied the potential contribution of precipitation to N runoff to water systems. High peaks of N concentration in runoff water (6.27–62.6 mg N L−1 ) and individual N runoff (1.42–18.9 kg N ha−1 ) from November to February of the next year (Fig. 6b) suggested that
Table 5 Contribution of drainages induced by diverse causes to total N runoff during the three rice seasons from 2007 to 2010. Crop season
Drainage eventsa Drainage before sowing
Drainage after precipitation
Drainage for mid-season aerations
2007 rice 2008 rice 2009 rice
34.5% 16.7% 8.00%
39.3% 55.8% 82.2%
26.2% 27.5% 9.80%
Average
19.7%
59.1%
21.2%
a
The electromagnetic flowmeter was used to precisely measure the amount of runoff water and differentiate the contributions of different causes to N runoff during rice season.
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
9
Fig. 6. N concentrations in runoff water and cumulative runoff N in paddy field during the three rice (a) and wheat (b) seasons from 2007 to 2010. The arrow denotes the timing of N fertilization.
N runoff mostly occurred in the early stage of wheat growth, since all required N fertilizer was applied during this stage (Table 1). The calculated N runoff during this period comprised an even greater percentage (87–95%) of the total N runoff in wheat season. NO3 − predominated over NH4 + in the runoff during wheat seasons (Fig. 6b; Table 4). One important reason might be that a great amount of NO3 − from N fertilizer accumulated in the cultivated layer (0–15 cm) could be transported horizontally into the 15 cm
drainage ditches and flow out of the field on rainy days (Fig. 2). Chen et al. (2003) investigated the characteristics of nitrate horizontal transport in paddy soil, and found that the horizontal transport of NO3 − was strongly correlated with the soil moisture content. Using the lysimeters (Fig. 3c), we were able to separate N runoff on the soil surface from that at 15 cm soil depth during two consecutive wheat growing seasons. The results interestingly revealed that on average, 84% of N runoff occurred at the 15 cm depth, with only 16%
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X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
Table 6 Contribution value of N loads in runoff and leaching from paddy soils under rice-wheat rotation. Annual leaching (kg N ha−1 )
Rotation
Crop season
Seasonal runoff + leaching (kg N ha−1 )
Annual total runoff + leaching (kg N ha−1 )
Annual runoff (kg N ha−1 )
2007/2008
2007 rice 2007/2008 wheat
25.3 (42.7%)a 33.9 (57.3%)
59.2
55.2 (93.2%)
3.99 (6.8%)
2008/2009
2008 rice 2008/2009 wheat
8.47 (15.3%) 46.8 (84.7%)
55.3
45.5 (82.3%)
9.84 (17.7%)
2009/2010
2009 rice 2009/2010 wheat
24.0 (25.8%) 69.1 (74.2%)
93.1
77.9 (83.7%)
69.2 ± 20.6
59.5 ± 16.6 (86.0%)
Average a
15.2 (16.3%) 9.68 ± 5.61 (14.0%)
Contribution percentage to annual total runoff + leaching shown in parentheses.
on the surface. This is because the horizontal transport of water in the top 15 cm soil layer preceded the surface flow during the gradual saturation of soil by rainwater. Obviously, the use of conventional drainage ditches greatly increased the risk of runoff loss for accumulated N in the cultivated layer, especially NO3 − , even though this practice was certainly effective in preventing waterlogging injury to wheat plants after frequent rain events. Further investigations are needed on this phenomenon, since it has not been mentioned in most previous studies. The quantities of accumulative N runoff (33.4–58.7 kg N ha−1 ; Table 4) from the three wheat seasons were far greater than those from the corresponding rice seasons (2.65–21.8 kg N ha−1 ), even though N applied in wheat season was 100 kg N ha−1 less than that in rice season. The data were also higher than the field measurements reported by Guo et al. (14.7 kg N ha−1 ; 2004b) and Tian et al. (5.2–38.6 kg N ha−1 ; 2007) in the same region. These results suggest that runoff driven by precipitation and drainage ditches is a very important pathway for N export from paddy soil during wheat season. 3.5. The rice-wheat system is a potential N source for water pollution The paddy field ecosystem has been usually considered an environment-friendly anthropogenic wetland ecosystem (Gong, 1985; Greenland, 1997; Xu et al., 1998). Although rice paddy fields with little N fertilizer can serve as a sink for N when irrigated with N-polluted water (Breen, 1990; Ou et al., 1992; Takeda et al., 1997), paddy fields under high N fertilization and improper water management were a potential source for N pollution of water. In the current field-scale investigation that examined rice-wheat paddy soil from 2007 to 2010, the annual N load in runoff and leaching was as high as 55.3–93.1 kg N ha−1 , with a mean of ca. 69.2 kg N ha−1 (Table 6). After deducting the annual averaged N load in the irrigated water (mainly in rice season) of 11.8 kg N ha−1 in the same field (Zhao et al., 2012), it was calculated that the annual export of N via water movement from an integral rice-wheat rotation was 57.4 kg N ha−1 . This value is still greater than most previously reported values from paddy fields in the same region (Gao et al., 2004; Guo et al., 2004a,b; Liang et al., 2010; Tian et al., 2007; Xie et al., 2007; Xing et al., 2001; Zhu et al., 1997, 2000, 2003). This considerable N export via water flow in fact was primarily attributed to high N fertilization and heavy water runoff driven by diverse artificial drainage practices in rice and wheat seasons. Besides N export via water flow, gaseous emission is another important N export from rice-wheat rotated paddy field with a substantial environmental cost. Our previous observation of N balance showed gaseous N export via denitrification (22% of total N output) and ammonia volatilization (17%) was greater than N export via runoff and leaching (Zhao et al., 2012). However, N export via water flow has more direct and greater effects on water pollution in comparison with gaseous N export into the atmosphere. Because
high N fertilization and frequent flooding-draining alternation is popular rice-wheat rotation cultivation patterns in the TLR, the contribution of N export via runoff and leaching from rice-wheat rotation to water N pollution in this region should not be neglected from viewpoint of mitigation of agricultural non-point-source water pollution. The TLR has exhibited increasing N deposition since the 1980s (Zhao et al., 2009b). Atmospheric N deposition investigated at the monitoring site averaged 30.5 kg N ha−1 year−1 from 2007 to 2010 (Zhao et al., 2012). Therefore, besides irrigation N load and fertilization, atmospheric deposition of N certainly contributed somewhat to the 69.2 kg N ha−1 of outflow N load via runoff and leaching from the rice-wheat rotation system in the current study. Tian et al. (2011) recently reported that approximately 14% of wet deposition of N was lost to runoff during wheat season. However, it is still very difficult to measure the accurate proportion of N deposition to total N outflow from the integrated rice-wheat rotation system, due to the lack of a proper method for distinguishing the amounts of N contributed by deposition, biological fixation, mineralization, and fertilization to N runoff and leaching. So far there have been few systematic and comprehensive in situ studies with long-term monitoring in the TLR that investigated the contribution of rice production under the conventional rice-wheat rotation system to N pollution of ground and surface waters. We have attempted to estimate N runoff and leaching with a three-year-round consecutive field observation (6 seasons) and the methodological improvements as mentioned previously (Figs. 2 and 3). However, it should be noted that certain errors in the measurement of N leaching remain, mainly due to the lack of uniform and more direct metering methods for the volume of leaching water in rice and wheat seasons. Nevertheless, the reliability of observed N runoff during rice and wheat seasons could be warranted, because we used electromagnetic flowmeters to accurately measure the water volume of individual runoff events (and also irrigation events in rice season) for both rice and wheat seasons on the field scale. Since the results showed that the overwhelming majority of the total N export via water flow came from N runoff (82–93%; Table 6), we considered the limitations in techniques for measurement of leaching water should not invalidate the conclusion that rice-wheat rotation under conventional N and water management is a significant potential N source for water pollution in the TLR. 4. Conclusion We conducted a three-year-round consecutive field observation (6 seasons) of N export via water with improved metering methods for runoff and leaching water. The net export of N from an integral rice-wheat rotation can be as high as 57.4 kg N ha−1 after deducting N load in the irrigation water. This value was greater than that reported by most previous studies. The highly fertilized and intensively flooded/drained rice-wheat rotation in fact serves
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
as a significant potential N source for water pollution in the TLR. The considerable N runoff in wheat season is mainly attributed to acceleration of water flow by the drainage ditches and moderately heavy rainfall, while a large amount of N runoff loss during rice season is due to frequent artificial draining. Therefore, controlling N runoff in both wheat season and rice season is very important for controlling water pollution from paddy soil. Acknowledgements We greatly thank editors and anonymous referees for their valuable comments that have greatly improved the manuscript. We specially thank Prof. Xiaoyuan Yan and Dr. Chaopu Ti in ISS, CAS for technical support on artwork. This study was funded by the National Natural Science Foundation of China (grant 41001147), the Knowledge Innovation Program of the Chinese Academy of Sciences (grant KSCX2-EW-B-1-3), and Special Fund for Agroscientific Research in the Public Interest (grant 200903011). References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration- guidelines for computing crop water requirements-FAO Irrigation and Drainage Paper No. 56. FAO, Rome. Breen, P.F., 1990. A mass balance method for assessing the potential of artificial wetlands for wastewater treatment. Water Res. 24, 689–697. Chen, X.M., Shen, Q.R., Pan, G.X., Liu, Z.P., 2003. Characteristics of nitrate horizontal transport in a paddy field of the Tai Lake region, China. Chemosphere 50, 703–706. Gao, C., Zhu, J.G., Zhu, J.Y., Gao, X., Dou, Y.J., Hosen, Y., 2004. Nitrogen export from an agriculture water shed in the Taihu Lake area, China. Environ. Geochem. Hlth. 26, 199–207. Gong, Z.T., 1985. Wetland soils in China. In: Wetland: Characterization Classification and Utilization. International Rice Research Institute, Philippines, pp. 473–487. Greenland, D.J., 1997. The Sustainablility of Rice Farming. CAB International, Wallingford, UK. Guo, H.Y., Wang, X.R., Zhu, J.G., 2004a. Quantification and index of non-point source pollution in Taihu Lake Region. Environ. Geochem. Hlth 26, 147–156. Guo, H.Y., Zhu, J.G., Wang, X.R., Wu, Z.H., Zhang, Z., 2004b. Case study on nitrogen and phosphorus emissions from paddy field in Taihu region. Environ. Geochem. Hlth. 26, 209–219. International Rice Research Institute, 1987. Physical measurements in flooded rice soil. The Japanese Methodologies, pp. 42–44. Ju, X.T., Xing, G.X., Chen, X.P., Zhang, S.L., Zhang, L.J., Liu, X.J., Cui, Z.L., Yin, B., Christie, P., Zhu, Z.L., Zhang, F.S., 2009. Reducing environmental risk by improving N management in intensive Chinese agricultural systems. PANS 106, 3041–3046. Kim, J.S., Oh, S.Y., Oh, K.Y., 2006. Nutrient runoff from a Korean rice paddy water shed during multiple storm events in the growing season. J. Hydrol. 327, 128–139. Li, Q.K. (Ed.), 1992. Paddy soils of China. Science Press, Beijing (in Chinese). Liang, X.Q., Xu, L., Li, H., He, M.M., Qian, Y.C., Liu, J., Nie, Z.Y., Ye, Y.S., Chen, Y.X., 2010. Influence of N fertilization rates, rainfall, and temperature on nitrate leaching from a rained winter wheat field in Taihu water shed. Phys. Chem. Earth, http://dx.doi.org/10.1016/j.pce.2010.03.017.
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Ou, Z., Chang, S., Gao, Z., Sun, T., Qi, E., Ma, X., Li, P., Zhang, L., Yang, G., Han, S., Song, Y., Zhang, H., Ren, L., Qu, X., 1992. Paddy rice slow-rate land treatment system II. Hydraulic balances and results of 4 years’ operation. Water Res. 26, 1487–1494. Pandey, S., Mortimer, M., Wade, L., Tuong, T.P., Lopez, K., Hardy, B., 2002. Direct seeding: research strategies and opportunities. In: Proceeding of the International Workshop on Direct seeding in Asian Rice Systems: Strategic Research Issues and Opportunities, 25–28 January 2000, Bangkok, Thailand. International Rice ˜ (Philippines). Research Institute, Los Baosnos Takeda, I., Fukushima, A., Tanaka, R., 1997. Non-point pollutant reduction in a paddyfield watershed using a circular irrigation system. Water Res. 31, 2685–2692. Tian, Y.H., Ying, B., Yang, L.Z., Yin, S.X., Zhu, Z.L., 2007. Nitrogen runoff and leaching losses during rice-wheat rotation in Taihu Lake Region, China. Pedosphere 17, 445–456. Tian, Y.H., Yang, L.Z., Ying, B., Zhu, Z.L., 2011. Wet deposition of N and its runoff flow during wheat seasons in the Tai Lake Region, China. Agric. Ecosyst. Environ. 141, 224–249. Wang, S.Q., Zhao, X., Xing, G.X., Shi, W.M., Yang, L.Z., Zhang, Z.H., 2011. An improved field undisturbed tension-free monolith lysimeter with water supply system for approximately simulating groundwater recharge and field situations. China Patent. C.N. 201120086263.0. Date issued: 23, November. Xie, Y.X., Xiong, Z.Q., Xing, G.X., Sun, G.Q., Zhu, Z.L., 2007. An assessment of nitrogen pollutant sources in surface water in Taihu region. Pedosphere 17, 200–208. Xing, G.X., Cao, Y.C., Shi, S.L., Sun, G.Q., Du, L.J., Zhu, J.G., 2001. N pollution sources and denitrification in waterbodies in Taihu Lake region. Sci. China Ser. B 44, 304–314. Xing, G.X., Cao, Y.C., Shi, S.L., Sun, G.Q., Du, L.J., Zhu, J.G., 2002. Denitrification in underground saturated soil in a rice paddy region. Soil Biol. Biochem. 34, 1593–1598. Xu, Q., Yang, L.Z., Dong, Y.H. (Eds.), 1998. Rice field ecological system of China. Chinese Agricultural Press, Beijing. Yoshinaga, I., Miura, A., Hitomi, T., Hamada, K., Shiratani, E., 2007. Runoff nitrogen from a large sized paddy field during a crop period. Agric. Water Manage. 87, 217–222. Zhang, Y.H., Hu, C.S., Zhang, J.B., Chen, D.L., Li, X.X., 2005. Nitrate leaching in an irrigated wheat-maize rotation field in the North China Plain. Pedosphere 15, 196–203. Zhao, X., Xie, Y.X., Xiong, Z.Q., Yan, X.Y., Xing, G.X., Zhu, Z.L., 2009a. Nitrogen fate and environmental consequence in paddy soil under rice-wheat rotation in the Tainhu lake region, China. Plant Soil 319, 225–234. Zhao, X., Yan, X.Y., Xiong, Z.Q., Xie, Y.X., Xing, G.X., Shi, S.L., Zhu, Z.L., 2009b. Spatial and temporal variation of inorganic nitrogen wet deposition to the Yangtze River Delta Region, China. Water Air Soil Pollut. 203, 277–289. Zhao, X., Min, J., Wang, S.Q., Shi, W.M., Xing, G.X., 2011. Further understanding of nitrous oxide emission from paddy fields under rice/wheat rotation in south China. J. Geophys. Res., 116, Go2016, http://dx.doi.org/10.1029/2010JG001528. Zhao, X., Zhou, Y., Wang, S.Q., Xing, G.X., Shi, W.M., Xu, R.K., Zhu, Z.L., 2012. Nitrogen balance in a highly fertilized rice–wheat double-cropping system in Southern China. Soil Sci. Soc. Am. J., http://dx.doi.org/10.2136/sssaj2011.0236. Zhu, J.G., Han, Y., Liu, G., Zhang, Y.L., Shao, X.H., 2000. Nitrogen in percolation water inpaddy fields with a rice/wheat rotation. Nutr. Cycl. Agroecosyst. 57, 75–82. Zhu, J.G., Liu, G., Han, Y., Zhang, Y.L., Xing, G.X., 2003. Nitrate distribution and denitrification in the saturated zone of paddy field under rice/wheat rotation. Chemosphere 50, 725–732. Zhu, Z.L., Wen, Q.X., Freney, J.R. (Eds.), 1997. Nitrogen in Soils of China. Kluwer Academic Publishers, Dordrecht, pp. 239–279. Zhu, Z.L., Chen, D.L., 2002. Nitrogen fertilizer use in China-contributions to food production impacts on the environment and best management strategies. Nutr. Cycl. Agroecosyst. 63, 117–127. Zhu, Z.L., Norse, D., Sun, B. (Eds.), 2006. Policy for Reducing Non-point Pollution from Crop Production in China. Chinese Environmental Science Press, Beijing.
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Nitrogen runoff dominates water nitrogen pollution from rice-wheat rotation in the Taihu Lake region of China Xu Zhao a , Yang Zhou b , Ju Min a , Shenqiang Wang a,∗ , Weiming Shi a , Guangxi Xing a,∗ a State Key Laboratory of Soil and Sustainable Agriculture, Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China b Resource and Environmental College, Anhui Agricultural University, Hefei 230036, China
a r t i c l e
i n f o
Article history: Received 8 December 2011 Received in revised form 23 April 2012 Accepted 25 April 2012 Keywords: Rice-wheat rotation Runoff Leaching Water pollution Conventional management
a b s t r a c t Nitrogen (N) runoff and leaching are two direct pathways of N pollution from agricultural land to water systems. It is not yet conclusive whether intensive rice-wheat rotation system contributes to N pollution in the surrounding water systems in the Taihu Lake region (TLR). We conducted a three-year field experiments to monitor N runoff and leaching from rice-wheat paddy soil. Annual N runoff and leaching was in the range of 55.3–93.1 kg N ha−1 , with a mean of ca. 69.2 kg N ha−1 . Of the total N export to water systems, 82–93% was from N runoff and only 7–18% was from N leaching. For seasonal distribution, the wheat season contributed 57–85% of total N, while the rice season contributed 15–43%. Because of different water practices, N runoff and leaching exhibited completely different patterns in two crop seasons. Considerable N runoff in wheat season can mainly be attributed to moderately heavy rainfall and accelerated water flow in existing drainage ditches. Large variations in N runoff during rice season were predominantly due to variable natural precipitation and irregular artificial draining. N leaching appeared to be related to fertilization and precipitation in wheat season, while it was associated with fertilization and soil depth during rice season. These field data suggest that runoff dominated N export from rice-wheat rotation under conventional N and water management contributes potentially to water pollution in the TLR. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The Taihu Lake region (TLR) in the Yangtze River delta (Fig. 1) covers 36,500 km2 . As one of the five major rice production regions in China, TLR has 75% of its total land growing rice. Rice-wheat rotation has been practiced here for thousands of years (Li, 1992). This region is one of the most fertilized regions with nitrogen (N) in the world. Currently, the annual application rate of synthetic N fertilizers is as high as 500–600 kg N ha−1 for two crops (Xing et al., 2002). There is, therefore, growing concern about environmental impacts induced by the high N loads (Zhu et al., 2006). N runoff and leaching from rice-wheat paddy fields was not well documented until the late 1990s (Gao et al., 2004; Guo et al., 2004a,b; Ju et al., 2009; Liang et al., 2010; Tian et al., 2007; Zhao et al., 2009a; Zhu and Chen, 2002; Zhu et al., 2000). Under current standard farming conditions in the TLR, the rice and
∗ Corresponding authors. Tel.: +86 25 86881534; fax: +86 25 86881028. E-mail addresses: [email protected] (X. Zhao), [email protected] (S. Wang), [email protected] (G. Xing). 0167-8809/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agee.2012.04.024
wheat seasons follow a distinct water regime. Flooding the field alternating with frequent draining (e.g., for midseason aerations and harvest) is common practice during rice season. Contrarily, rainwater is the only source of soil moisture during wheat season (Zhao et al., 2011). Despite of less rainfall in wheat season than in rice season, it is still excessive for normal growth of wheat plants due to subtropical monsoon climate; drainage ditches are usually needed to protect the wheat plants from waterlogging injury. These distinct water schemes can influence the transformation and migration of N, and consequently cause great variations in runoff and leaching N losses between rice and wheat seasons. Our previous lysimeter study reported that N runoff and leaching are greater in wheat season than in rice season (Zhao et al., 2009a), although elaborate explanations for this phenomenon are not available. The variation of N runoff and leaching patterns in both crop seasons is also not well characterized. So far, it is still unclear whether intensive rice production is one of major contributors to water N pollution in the TLR. Because of the existence of field levees and plough pans with low permeability, N runoff and leaching from paddy soil are considered to be minimal (Zhu et al., 1997). Their contribution to water pollution is often insignificant in comparison with that from the increasing amount
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Fig. 1. Location of the Taihu Lake region (TLR) in China. The star denotes the monitoring site.
of human and animal excreta (Xing et al., 2001) and atmospheric N deposition (Xie et al., 2007). However, a number of studies have concluded that the large inputs of synthetic N fertilizer used in rice production are one of the major sources of N pollution in water bodies (Gao et al., 2004; Guo et al., 2004a,b; Liang et al., 2010; Tian et al., 2007; Zhu et al., 2000, 2003). One likely explanation for the divergent results is that appropriate methods for in situ observation of N runoff and leaching are lacking. Although N concentrations in runoff and leaching water can be accurately determined, the amount of water flow is not actually measured in paddy field. It is usually large-scale estimates by water balance methods using multifarious parameters from soil hydroconductibility coefficients, soil evaporation, crop transpiration, irrigation, precipitation, etc. (Allen et al., 1998; Tian et al., 2007; Zhu et al., 2000), which may not reflect the true conditions of the micro-environment. Consequently, an amount of uncertainty is introduced into the estimates of N exports via runoff and leaching. In addition, previous studies have been mostly based on short-term observations of either a crop rotation or a single N pathway (i.e., either runoff or leaching) from either small plots or monolith lysimeters. Lack of consecutive year-round investigations on both N runoff and leaching one time at field scale might be another alternative explanation for the current divergent results of N direct export to water systems. Since June 2007, we had conducted a three-year-round consecutive field-scale observation to measure total N input and N output of rice-wheat rotation agroecosystem with improved metering methods for water flow, including irrigation, runoff and leaching. The entire N balance and its contributing input/output factors in this high-N-input agricultural system were discussed separately (Zhao et al., 2012). The emphasis of this paper therefore was on variation patterns in N runoff and leaching and their key factors. We aim to examine how much the potential contribution of direct N export is from intensive rice-wheat rotation system to water N pollution, and illustrate how the conventional water management affects N export via water flow in rice and wheat seasons. The information could reveal insights for reducing non-point pollution of water from paddy field in TLR.
2. Materials and methods 2.1. Study area and rice field plot experiment The experiment was located in the midwest of the TLR, on the northwest side of Taihu Lake, within 1 km of the shore (Fig. 1). This location has a subtropical monsoon climate with an average temperature of 15.7 ◦ C and an annual rainfall of 1177 mm. The paddy soil is classified as Gleyi–Stagnic Anthrosols, and originated from a Lacustrine deposit as parent material. The soil consists of 8.30% sand, 81.5% silt, and 10.2% clay by volume, and contains 15.4 g kg−1 organic C, 1.79 g kg−1 N, and 11.8 cmol kg−1 CEC. The pH (H2 O) of the soil (0–15 cm) was 5.6. A piece of paddy field (Fig. 2) belonging to a local farmer was selected for observation. It is surrounded by large paddy fields cultivated by other local farmers. The paddy field was treated with urea at rates of 300 kg N ha−1 for rice season and 200 kg N ha−1 for wheat season. For N application, 30% was basally applied, 40% was top-dressed at the tillering stage, and the remaining 30% was topdressed at the ear differentiation stage for each crop. Phosphate and potassium fertilizers were applied basally in the form of superphosphate at a rate of 60 kg P2 O5 ha−1 and in the form of potassium chloride at a rate of 45 kg K2 O ha−1 . In this area, the following normal cultivation and field management practices in the TLR are usually adopted in the ricewheat rotated paddy fields. Firstly, approximately 240–300 and 200–250 kg N ha−1 of chemical N fertilizers are routinely applied to the soil to grow rice in summer and wheat in winter, respectively. Secondly, direct sowing and surface fertilizer application are employed due to convenience and economic costs in practice. Thirdly, the current standard water regime is popularly practiced. During rice seasons, flooded water was mostly maintained at a depth of 3–5 cm in the field except for drainage before sowing, several midseason aerations and final drainage events. At the beginning of each wheat season, drainage ditches (15 cm in depth and 10 cm in width; were usually spaced 1.5–2 m apart) were opened mechanically in the field (Fig. 2). In addition, a local crop grower controlled above-mentioned normal cultivation and field
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3
Fig. 2. Plane figure of the paddy field and landscape of drainage ditches during the period of wheat growth. Those drainage ditches were usually 15 cm in depth and 10 cm in width with 1.5–2 m apart from each other. This conventional practice can effectively prevent waterlogging that might injury wheat plants due to frequent rain events during the period of wheat growth.
management practices in the experimental field (Table 1). Thus, the results of the study were expected to be appropriately representative of the paddy fields under conventional farming in the TLR. 2.2. Sampling and analysis Porous pipes made of polyvinyl chloride were installed in triplicate at different soil depths to collect percolation water (Fig. 3a). Table 1 Dates of cultivation and management during the three rice-wheat rotations from 2007 to 2010. Crop rotation
Activity
2007 rice
Preflooding, June 6, 2007; Basal fertilization, June 8; Draining and sowing, June 9; Re-flooding, June 19; First top-dressing, June 25; Mid-season aeration, July 18–28; Second-top dressing, July 30; Mid-season aeration, August 4–8; Mid-season aeration, August 10–21; Final drainage, October 9; Harvesting, October 28. Sowing, November 14, 2007; basal fertilization, November 14; First top-dressing, December 15; Second top-dressing, February 7, 2008; Harvesting, May 24. Pre-flooding, June 11, 2008; Draining and sowing, June 14; Re-flooding, July 1; Basal fertilization, July 7; First top-dressing, July 21; Mid-season aeration, July 27 to August 3; Second-top dressing, August 6; Mid-season aeration, August 16 to August 23; Final drainage, October 3; Harvesting, October 18 Sowing, November 19, 2008; Basal fertilization, November 23; First top-dressing, December 26; Second top-dressing, February 10, 2009; Harvesting, May 24. Pre-flooding, June 4, 2009; Draining and sowing, June 7; Re-flooding, June 14; Basal fertilization, June 18; First top-dressing, July 5; Mid-season aeration, July 18–20; Mid-season aeration, July 26 to August 3; Second-top dressing, August 4; Mid-season aeration, August 14–23; Mid-season aeration, August 27 to September 3; Final drainage, October 14; Harvesting, October 23. Sowing, November 5, 2009; Basal fertilization, November 8; First top-dressing, December 4; Second top-dressing, February 24, 2010; Harvesting, June 3.
2007/2008 wheat
2008 rice
2008/2009wheat
2009 rice
2009/2010 wheat
Percolation water was collected with a vacuum hand pump usually at 10-day intervals during rice season, but more frequently immediately after fertilizer application. Likewise, percolation water was collected after every rain event during wheat season. Water preexisting in the porous pipe was drained and discarded before sampling to avoid contamination. Runoff water in both seasons was sampled at drainage outlet point when every drainage event occurred. Samples of irrigation water in rice season were taken for analysis at irrigation inlet point (Fig. 2) when every irrigation event occurred. Percolation, runoff and irrigation water samples were stored in 250 ml plastic bottles, and immediately frozen without filtering at −20 ◦ C in a freezer until analysis. Concentrations of NH4 + , NO3 − , and total N (TN) in these water samples were analyzed with a continuous-flow N analyzer (Skalar, Netherlands, with analytic error ± 3.9% and low detection limit of 0.2 mg N L−1 ). Three improvements in methodology varying from indirect estimates using water balance method were explored in the current study to directly obtain the amount of water export: (1) two electromagnetic flowmeters (Fig. 2) were installed to precisely measure the water volume of individual irrigation and runoff events on the field scale; (2) a slightly modified rapid-response percolation meter (International Rice Research Institute, 1987; Fig. 3b) was designed to actually observe the volume of water transferred via leaching during rice season; (3) field undisturbed tension-free monolith lysimeters with a special design (Fig. 3c; Wang et al., 2011) were used to proximately obtain the volume of water transferred via leaching in wheat season. Cumulative N runoff = sum of amount of individual runoff × (m3 ha−1 ) × N concentration in runoff water (mg N L−1 ) × 0.001
Cumulative N leaching (kg N ha−1 ) = time-interval weighted N concentration at 100 cm soil depth (mg N L−1 ) × total amount of leaching water (m3 ha−1 ) × 0.001 where time-interval weighted N concentration is defined as sum [(individual N concentration (mg N L−1 ) × intervals between two adjacent samplings (days)]/the total growth time (days).
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Fig. 3. Schematics view of porous pipes (a), rapid-response percolation meter (b) and the field undisturbed tension-free monolith lysimeter (c). The percolation meter was a modification type based on the meter designed by International Rice Research Institute (1987).
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Table 2 Average N concentration in percolation water from different soil depths and estimation of N leaching in the three rice-wheat rotations from 2007 to 2010. Crop season
Soil depth (cm)
Average N concentration in percolation water (mg N L−1 )a NO3 −
NH4 +
Growth period (day)
Leaching waterb (m3 ha−1 )
Total N
N loads in leaching water (kg N ha−1 )c NO3 −
NH4 +
Total N
2007 rice
20–40 40–60 60–80 80–100
1.27 0.94 0.52 0.41
± ± ± ±
0.21 0.31 0.32 0.25
2.44 1.65 0.77 0.84
± ± ± ±
1.34 0.6 0.53 0.74
4.28 3.08 1.71 1.80
± ± ± ±
1.54 1.05 1.06 1.28
144
1920
0.79 ± 0.47
1.61 ± 1.43
3.46 ± 2.46
2008 rice
20–40 40–60 60–80 80–100
3.15 2.65 2.34 0.78
± ± ± ±
0.25 0.92 0.92 0.35
1.51 1.28 1.83 1.63
± ± ± ±
0.87 0.80 1.72 1.10
5.03 4.53 4.56 3.00
± ± ± ±
0.83 0.55 1.79 1.86
129
1940
1.51 ± 0.68
3.15 ± 2.14
5.82 ± 3.60
2009 rice
20–40 40–60 60–80 80–100
0.29 0.38 0.34 0.37
± ± ± ±
0.06 0.18 0.23 0.14
0.93 0.28 0.35 0.77
± ± ± ±
0.52 0.10 0.21 0.25
2.28 1.13 1.15 2.20
± ± ± ±
0.60 0.33 0.26 0.14
141
2180
0.81 ± 0.55
1.68 ± 0.55
4.79 ± 0.30
2007/2008 wheat
20–40 40–60 60–80 80–100
7.77 5.29 8.52 1.71
± ± ± ±
0.49 1.89 3.61 1.37
0.14 0.25 0.26 0.11
± ± ± ±
0.03 0.09 0.10 0.03
10.4 5.69 8.97 3.81
± ± ± ±
1.93 2.08 3.57 2.91
192
138
0.24 ± 0.19
0.02 ± 0.00
0.53 ± 0.40
2008/2009 wheat
20–40 40–60 60–80 80–100
15.7 12.6 7.47 14.9
± ± ± ±
11.0 3.48 3.29 8.63
0.28 0.29 0.20 0.31
± ± ± ±
0.12 0.10 0.02 0.32
20.7 15.6 8.37 17.5
± ± ± ±
16.0 4.97 3.25 9.50
186
230
3.42 ± 1.98
0.07 ± 0.07
4.02 ± 2.18
20–40 40–60 60–80 80–100
13.7 11.3 8.52 7.40
± ± ± ±
3.83 4.12 1.03 1.02
0.40 0.52 0.56 0.34
± ± ± ±
0.21 0.40 0.59 0.26
16.1 14.1 10.0 8.83
± ± ± ±
5.03 5.77 2.25 0.73
210
1180
8.73 ± 1.20
0.40 ± 0.31
2009/2010 wheat
10.4 ± 0.87
Date are time-interval weighted means ± SD of the three replicates from field measurements. b Amount of leaching water in rice season was estimated based on the vertical leaching rate of 2 mm day−1 during the flooded period; amount of leaching water in wheat season was obtained from the field undisturbed tension-free monolith lysimeter (with 38 cm in diameter,100 cm in height) which has the same conditions as field treatment. c Calculated as N concentration in percolation water from 100 cm soil depth multiplied by amount of leaching water. a
The total amount of leaching water in rice season is defined as the rate of surface water vertical percolation (mm day−1 ) × flooded periods (day) × 10. Flooding condition in most period of rice season provided an approximately constant rate of water moving through soil profile. Therefore, the rate of surface water vertical percolation in rice season during flooded periods was measured using the percolation meter as illustrated in Fig. 3b. Compared with the water balance method, this method significantly simplified the calculation of the leaching water amount and reduced the error. The amount of water that percolated vertically through the cylinder equaled the amount that passed through the transparent tube on the float. In the current study field, the rate of vertical percolation was averaged at 2 mm day−1 from 15 replicate measurements distributed well in the plot. Unlike rice season, rainfall is the only driver of water leaching in wheat season. Water leaching occurred randomly because the rainfall amount, intensity and frequency varied greatly. This made the observation of amount of leaching water so difficult in the field scale. So we used field undisturbed tension-free monolith lysimeters (Fig. 3c) to directly obtain the total amount of leaching water in wheat season. Three lysimeters were installed and cultivated exactly like the rice or wheat plants in the field experiment under the same water and fertilization management. A simulated underground water supply system could provide a 10 cm height water layer in the bottom of lysimeters and prevent drought cracking of the soil monolith caused by evaporation and transpiration. Two runoff outlets were also installed at 0 and 15 cm soil depths to simulate runoff on the soil surface and from the 15 cm deep drainage ditches. These special practices ensured that the amounts of leaching water obtained from the lysimeters were more realistically similar to those in the field. Although certain errors remain, the data obtained by lysimeters are at least measured values from
the field, varying from estimated values using the water balance method. One homemade quadrate rain sampler (0.49 m2 in area and 150 cm in height from the ground) was installed near the field to measure precipitation. Individual precipitation (mm) = rainwater collected in each precipitation event . 0.49 m2
3. Results and discussion 3.1. Characteristic variations of N leaching in relation to fertilization and soil depth during rice season For the three rice seasons, TN concentration in percolation water was always higher before July than after August (Fig. 4a). This implied that N leaching tended to occur in the early stages of rice growth, during which 70% of seasonal N fertilizer was applied as basal fertilizer and the first N top dressing (Table 1). However, the fluctuation intensity of TN concentration in percolation water declined with increased soil depth (Fig. 4a), suggesting that the effect of N fertilization on N leaching weakened with the depth of soil. Average TN concentration in percolation water from the top 40 cm of soil was far greater than that from deeper soil layers (Table 2). This indicated that a large amount of fertilizer N that leached from the cultivated layer (0–20 cm) was actually retained in the top 40 cm soil layer. For rice growing season, the rate of water moving through the 0–100 cm soil layer was approximately constant under flooding condition, so percentage distribution of percolated N in vertical soil profile can be calculated based on
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X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
Fig. 4. Variation of N concentrations in percolation water from different soil depths in paddy field during the three rice (a) and wheat (b) seasons from 2007 to 2010. The arrow denotes the timing of N fertilization.
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11 Table 3 Vertical distribution of percolate N in 20–100 cm paddy soil profile during the three rice seasons from 2007 to 2010. Crop season
Soil deptha 20–40 cm
40–60 cm
60–80 cm
80–100 cm
2007 rice 2008 rice 2009 rice
39.4% 29.4% 33.7%
28.3% 26.5% 16.8%
15.8% 26.6% 17.0%
16. 6% 17.5% 32.5%
Average
34.2%
23.9%
19.8%
22.2%
a
For rice growing season, the rate of water moving through the 0–100 cm soil layer was approximately constant under flooding condition, so percentage distribution of percolated N in vertical soil profile can be calculated based on average concentrations of TN in percolation water from each soil depth.
average concentrations of TN in percolation water from each soil depth. The results (Table 3) showed that 58% of percolated N accumulated in the top 60 cm soil layer, and approximately 78% of percolated N was in the top 80 cm soil layer in rice season. Apparently, not all percolated N from the cultivated layer entered into phreatic water. This raises an interesting question: to what depth can percolated N be considered as leaching loss when N leaching in the paddy field is determined using suction cups or Teflon tubes? At present, the reported results on N leaching loss from paddy soil in the TLR differ substantially due to differences in methodology (e.g., backfill soil column, suction cups or Teflon tubes, in situ lysimeter), estimation method, the standard of soil depth regarded as leaching loss (40–100 cm), etc. Therefore, a standard observation and calculation method for N leaching is expected to be found on a regional scale for the comparison of results in rice paddies and evaluation of N leaching loss to groundwater. The estimates of N loads in percolation water from 100 cm were 3.46 to 5.82 kg N ha−1 (Table 2) during the three rice seasons. Based on the current investigation and previous studies (Tian et al., 2007; Zhao et al., 2009a; Zhu et al., 2000), it can be concluded that N leaching from most flooded rice paddies in TLR may not be as high as feared, because less than 6 kg N ha−1 was leached, and therefore the effect on N pollution of groundwater might not necessarily be higher than that of some other upland soil (Zhang et al., 2005). One important reason for the low rate of N leaching might be the puddled layer with low permeability at about 15–20 cm in depth. The percolated NO3 − was minimal during the three rice seasons, except in the short stage of wheat-rice alternation (Fig. 4a and Table 2); at that time, it might actually have originated from
7
remnant NO3 − from the previous wheat season (Zhao et al., 2011). The low NO3 − concentration in percolation water was due to that flooding conditions during the rice seasons created an anaerobic soil environment for denitrification. The fact of excessive accumulation of NH4 + in percolated water indicated that after quick hydrolysis of applied urea with oxygen-limited nitrification under flooding conditions, NH4 + could still flow downward with infiltrated water (Fig. 4a). 3.2. Significant N leaching closely associated with precipitation and fertilization during wheat season Notable vertical movement of N existed in the soil profile during the three wheat seasons. This conclusion is supported by the following evidence: the 40 cm depth exhibited higher TN concentration values initially but lower values later than deeper soil depths at the later stages of wheat growth, and the high peaks of TN concentration were usually found in percolation water from 100 cm soil depth (Fig. 4b). The applications of fertilizer N did not always induce immediate peaks in N concentration in percolation water, implying that the majority of N that was not taken up by wheat plants might have accumulated in the cultivated soil layer after N fertilization. When a heavy rain event occurred, N could be flushed out and move downward through percolation and runoff. NO3 − leaching was the predominant form of N leaching during wheat season (Table 2 and Fig. 4b). The probable reason might be strong nitrification under aerobic soil conditions mainly created by the drainage ditches in wheat season. NO3 − originating from fertilization can accumulate in the cultivated soil layer and move downward through soil with water flow more easily than NH4 + due to its negative charge. Significant amounts of NH4 + could still migrate downward if intense storm events occurred immediately after the N application (as seen in the 2009/2010 wheat season; Fig. 4b). Rainfall was the significant factor in the leachate amount during wheat season. The precipitation amount, intensity, and frequency (Fig. 5) probably had important effects on the N concentration in percolation water (Fig. 4b) and might have caused great variations (0.53–10.4 kg N ha−1 ; Table 2) in the amounts of N leaching. Despite great variations in the amount of N leaching, the significant downward migration of NO3 − in the soil profile during wheat seasons suggests a risk of groundwater pollution in paddy soil. An investigation carried out in the 1990s in the TLR found that the concentration
Fig. 5. Precipitation and runoff water caused by precipitation during the three rice-wheat rotations from 2007 to 2010.
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X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
Table 4 N runoff in paddy soils during the three rice-wheat rotations from 2007 to 2010. Rotation
Crop season
Total water runoff a
Precipitation
N runoff b Total N
(mm)
NO3 − (kg N ha−1 )
NH4 +
(m3 ha−1 ) 2007/2008
2007 rice 2007/2008 wheat
4701 1918
661 434
4.89 24.5
13.2 6.88
21.8 33.4
2008/2009
2008 rice 2008/2009 wheat
1535 1991
533 442
0.41 27.3
0.77 11.1
2.65 42.8
2009/2010
2009 rice 2009/2010 wheat
8824 3020
1055 563
9.60 32.9
3.48 19.7
19.2 58.7
a b
Total water runoff was the sum of individual runoff volume measured by an electromagnetic flowmeter at the drainage outlet. N runoff was the sum of amount of individual runoff (m3 ha−1 ) × N concentration in runoff water (mg N L−1 ) × 0.001.
of NO3 − averaged 7.89 mg N L−1 among 40 well water samples, and revealed that NO3 − in well water in the TLR was commonly associated with N leaching from farmland (Xing et al., 2001). 3.3. Variations of runoff N in rice season affected by precipitation and artificial draining Fig. 6a shows the great interseasonal variation in runoff N during each rice season. This variation was predominantly governed by the times of fertilization and runoff events. If runoff happened to occur soon after N fertilization, high N concentration was inevitably present in the runoff water. A similar phenomenon has been observed by other scientists (Kim et al., 2006; Tian et al., 2007; Yoshinaga et al., 2007). However, if runoff was delayed for one week or more after N fertilization, N concentration was low and stable in the runoff water (as seen in the 2008 rice season). The interseasonal variation in the amount of individual N runoff was the result of the influence of N concentration and the amount of runoff water. The observed runoff N from rice paddy soil varied from 2.65 to 21.8 kg N ha−1 during the three rice seasons (Table 4). This variation was largely dependent of the difference in volume and the frequency of water runoff, which was in connection with precipitation and artificial drainage. High amount of N runoff in 2007 and 2009 rice seasons could be attributed to their frequent runoff events caused by artificial draining and precipitation. Low amounts of N runoff during the 2008 rice season was due to fewer runoff events under the water regime of intermittent irrigation with drained soil by natural evaporation and percolation. Artificial draining is one important factor dominating N runoff in rice season. Currently, direct seeding instead of transplanting has become a popular cultivation pattern due to its convenience and cost economics in practice (Pandey et al., 2002). A drainage event is usually conducted after steeping the field (pre-flooding) in order to provide drained soil for rice seed broadcasting and the emergence of rice seedlings (which usually takes 10–15 days). Drainage events also occur during several midseason aerations (MSA) to inhibit ineffective tillering, and again at the mature stage before rice harvest. N runoff loss is inevitable due to these frequent artificial drainages.
Natural precipitation is another important cause of runoff during the flooded period of rice growth. In a flooded rice paddy, runoff is closely correlated with rainfall and the height of the field levees (Zhao et al., 2009a). For the study area, it is rare that rainfall would exceed 100 mm in rice season, so runoff directly through field levees seems impossible due to the height of the field levees (15–20 cm above soil surface). However, it is common practice to lower surface water in rainy days to prevent the inhibition effect of deep flooded water on root growth and plant tillering. The usual practice is to reduce the height of the drainage outlet to 5–7 cm, which may allow significant runoff if rainwater substantially exceeds the height of the drainage outlet due to a heavy rainfall event. We used an electromagnetic flowmeter (Fig. 2a) to precisely measure the amount of runoff water and firstly differentiate the contributions from different sources to N runoff during rice season. The average data from the three rice seasons showed that 59.1% of seasonal N runoff was caused by precipitation, 19.7% by drainage before sowing, and 21.2% by midseason aerations (Table 5). This was not consistent with the common knowledge that N runoff from paddy soil during rice season was minimal because of the existed field levees (Zhao et al., 2009a; Zhu et al., 1997). The reason was largely due to frequent artificial draining induced by diverse causes.
3.4. Considerable N runoff in wheat season attributed to existing drainage ditches and moderately heavy rainfall The variations in runoff amounts exhibited a pattern similar to the pattern of rainfall during the three wheat seasons (Fig. 5). Apparently, precipitation was the main factor governing the runoff. However, the runoff events were not completely synchronized with each rainfall event, but tended to happen immediately after successive rainfall events and heavy rainfall events (Fig. 5). 44–57% of the seasonal rainfall during the three wheat seasons was lost via drainage in the wheat fields (Table 4). This fact implied the potential contribution of precipitation to N runoff to water systems. High peaks of N concentration in runoff water (6.27–62.6 mg N L−1 ) and individual N runoff (1.42–18.9 kg N ha−1 ) from November to February of the next year (Fig. 6b) suggested that
Table 5 Contribution of drainages induced by diverse causes to total N runoff during the three rice seasons from 2007 to 2010. Crop season
Drainage eventsa Drainage before sowing
Drainage after precipitation
Drainage for mid-season aerations
2007 rice 2008 rice 2009 rice
34.5% 16.7% 8.00%
39.3% 55.8% 82.2%
26.2% 27.5% 9.80%
Average
19.7%
59.1%
21.2%
a
The electromagnetic flowmeter was used to precisely measure the amount of runoff water and differentiate the contributions of different causes to N runoff during rice season.
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
9
Fig. 6. N concentrations in runoff water and cumulative runoff N in paddy field during the three rice (a) and wheat (b) seasons from 2007 to 2010. The arrow denotes the timing of N fertilization.
N runoff mostly occurred in the early stage of wheat growth, since all required N fertilizer was applied during this stage (Table 1). The calculated N runoff during this period comprised an even greater percentage (87–95%) of the total N runoff in wheat season. NO3 − predominated over NH4 + in the runoff during wheat seasons (Fig. 6b; Table 4). One important reason might be that a great amount of NO3 − from N fertilizer accumulated in the cultivated layer (0–15 cm) could be transported horizontally into the 15 cm
drainage ditches and flow out of the field on rainy days (Fig. 2). Chen et al. (2003) investigated the characteristics of nitrate horizontal transport in paddy soil, and found that the horizontal transport of NO3 − was strongly correlated with the soil moisture content. Using the lysimeters (Fig. 3c), we were able to separate N runoff on the soil surface from that at 15 cm soil depth during two consecutive wheat growing seasons. The results interestingly revealed that on average, 84% of N runoff occurred at the 15 cm depth, with only 16%
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X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
Table 6 Contribution value of N loads in runoff and leaching from paddy soils under rice-wheat rotation. Annual leaching (kg N ha−1 )
Rotation
Crop season
Seasonal runoff + leaching (kg N ha−1 )
Annual total runoff + leaching (kg N ha−1 )
Annual runoff (kg N ha−1 )
2007/2008
2007 rice 2007/2008 wheat
25.3 (42.7%)a 33.9 (57.3%)
59.2
55.2 (93.2%)
3.99 (6.8%)
2008/2009
2008 rice 2008/2009 wheat
8.47 (15.3%) 46.8 (84.7%)
55.3
45.5 (82.3%)
9.84 (17.7%)
2009/2010
2009 rice 2009/2010 wheat
24.0 (25.8%) 69.1 (74.2%)
93.1
77.9 (83.7%)
69.2 ± 20.6
59.5 ± 16.6 (86.0%)
Average a
15.2 (16.3%) 9.68 ± 5.61 (14.0%)
Contribution percentage to annual total runoff + leaching shown in parentheses.
on the surface. This is because the horizontal transport of water in the top 15 cm soil layer preceded the surface flow during the gradual saturation of soil by rainwater. Obviously, the use of conventional drainage ditches greatly increased the risk of runoff loss for accumulated N in the cultivated layer, especially NO3 − , even though this practice was certainly effective in preventing waterlogging injury to wheat plants after frequent rain events. Further investigations are needed on this phenomenon, since it has not been mentioned in most previous studies. The quantities of accumulative N runoff (33.4–58.7 kg N ha−1 ; Table 4) from the three wheat seasons were far greater than those from the corresponding rice seasons (2.65–21.8 kg N ha−1 ), even though N applied in wheat season was 100 kg N ha−1 less than that in rice season. The data were also higher than the field measurements reported by Guo et al. (14.7 kg N ha−1 ; 2004b) and Tian et al. (5.2–38.6 kg N ha−1 ; 2007) in the same region. These results suggest that runoff driven by precipitation and drainage ditches is a very important pathway for N export from paddy soil during wheat season. 3.5. The rice-wheat system is a potential N source for water pollution The paddy field ecosystem has been usually considered an environment-friendly anthropogenic wetland ecosystem (Gong, 1985; Greenland, 1997; Xu et al., 1998). Although rice paddy fields with little N fertilizer can serve as a sink for N when irrigated with N-polluted water (Breen, 1990; Ou et al., 1992; Takeda et al., 1997), paddy fields under high N fertilization and improper water management were a potential source for N pollution of water. In the current field-scale investigation that examined rice-wheat paddy soil from 2007 to 2010, the annual N load in runoff and leaching was as high as 55.3–93.1 kg N ha−1 , with a mean of ca. 69.2 kg N ha−1 (Table 6). After deducting the annual averaged N load in the irrigated water (mainly in rice season) of 11.8 kg N ha−1 in the same field (Zhao et al., 2012), it was calculated that the annual export of N via water movement from an integral rice-wheat rotation was 57.4 kg N ha−1 . This value is still greater than most previously reported values from paddy fields in the same region (Gao et al., 2004; Guo et al., 2004a,b; Liang et al., 2010; Tian et al., 2007; Xie et al., 2007; Xing et al., 2001; Zhu et al., 1997, 2000, 2003). This considerable N export via water flow in fact was primarily attributed to high N fertilization and heavy water runoff driven by diverse artificial drainage practices in rice and wheat seasons. Besides N export via water flow, gaseous emission is another important N export from rice-wheat rotated paddy field with a substantial environmental cost. Our previous observation of N balance showed gaseous N export via denitrification (22% of total N output) and ammonia volatilization (17%) was greater than N export via runoff and leaching (Zhao et al., 2012). However, N export via water flow has more direct and greater effects on water pollution in comparison with gaseous N export into the atmosphere. Because
high N fertilization and frequent flooding-draining alternation is popular rice-wheat rotation cultivation patterns in the TLR, the contribution of N export via runoff and leaching from rice-wheat rotation to water N pollution in this region should not be neglected from viewpoint of mitigation of agricultural non-point-source water pollution. The TLR has exhibited increasing N deposition since the 1980s (Zhao et al., 2009b). Atmospheric N deposition investigated at the monitoring site averaged 30.5 kg N ha−1 year−1 from 2007 to 2010 (Zhao et al., 2012). Therefore, besides irrigation N load and fertilization, atmospheric deposition of N certainly contributed somewhat to the 69.2 kg N ha−1 of outflow N load via runoff and leaching from the rice-wheat rotation system in the current study. Tian et al. (2011) recently reported that approximately 14% of wet deposition of N was lost to runoff during wheat season. However, it is still very difficult to measure the accurate proportion of N deposition to total N outflow from the integrated rice-wheat rotation system, due to the lack of a proper method for distinguishing the amounts of N contributed by deposition, biological fixation, mineralization, and fertilization to N runoff and leaching. So far there have been few systematic and comprehensive in situ studies with long-term monitoring in the TLR that investigated the contribution of rice production under the conventional rice-wheat rotation system to N pollution of ground and surface waters. We have attempted to estimate N runoff and leaching with a three-year-round consecutive field observation (6 seasons) and the methodological improvements as mentioned previously (Figs. 2 and 3). However, it should be noted that certain errors in the measurement of N leaching remain, mainly due to the lack of uniform and more direct metering methods for the volume of leaching water in rice and wheat seasons. Nevertheless, the reliability of observed N runoff during rice and wheat seasons could be warranted, because we used electromagnetic flowmeters to accurately measure the water volume of individual runoff events (and also irrigation events in rice season) for both rice and wheat seasons on the field scale. Since the results showed that the overwhelming majority of the total N export via water flow came from N runoff (82–93%; Table 6), we considered the limitations in techniques for measurement of leaching water should not invalidate the conclusion that rice-wheat rotation under conventional N and water management is a significant potential N source for water pollution in the TLR. 4. Conclusion We conducted a three-year-round consecutive field observation (6 seasons) of N export via water with improved metering methods for runoff and leaching water. The net export of N from an integral rice-wheat rotation can be as high as 57.4 kg N ha−1 after deducting N load in the irrigation water. This value was greater than that reported by most previous studies. The highly fertilized and intensively flooded/drained rice-wheat rotation in fact serves
X. Zhao et al. / Agriculture, Ecosystems and Environment 156 (2012) 1–11
as a significant potential N source for water pollution in the TLR. The considerable N runoff in wheat season is mainly attributed to acceleration of water flow by the drainage ditches and moderately heavy rainfall, while a large amount of N runoff loss during rice season is due to frequent artificial draining. Therefore, controlling N runoff in both wheat season and rice season is very important for controlling water pollution from paddy soil. Acknowledgements We greatly thank editors and anonymous referees for their valuable comments that have greatly improved the manuscript. We specially thank Prof. Xiaoyuan Yan and Dr. Chaopu Ti in ISS, CAS for technical support on artwork. This study was funded by the National Natural Science Foundation of China (grant 41001147), the Knowledge Innovation Program of the Chinese Academy of Sciences (grant KSCX2-EW-B-1-3), and Special Fund for Agroscientific Research in the Public Interest (grant 200903011). References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration- guidelines for computing crop water requirements-FAO Irrigation and Drainage Paper No. 56. FAO, Rome. Breen, P.F., 1990. A mass balance method for assessing the potential of artificial wetlands for wastewater treatment. Water Res. 24, 689–697. Chen, X.M., Shen, Q.R., Pan, G.X., Liu, Z.P., 2003. Characteristics of nitrate horizontal transport in a paddy field of the Tai Lake region, China. Chemosphere 50, 703–706. Gao, C., Zhu, J.G., Zhu, J.Y., Gao, X., Dou, Y.J., Hosen, Y., 2004. Nitrogen export from an agriculture water shed in the Taihu Lake area, China. Environ. Geochem. Hlth. 26, 199–207. Gong, Z.T., 1985. Wetland soils in China. In: Wetland: Characterization Classification and Utilization. International Rice Research Institute, Philippines, pp. 473–487. Greenland, D.J., 1997. The Sustainablility of Rice Farming. CAB International, Wallingford, UK. Guo, H.Y., Wang, X.R., Zhu, J.G., 2004a. Quantification and index of non-point source pollution in Taihu Lake Region. Environ. Geochem. Hlth 26, 147–156. Guo, H.Y., Zhu, J.G., Wang, X.R., Wu, Z.H., Zhang, Z., 2004b. Case study on nitrogen and phosphorus emissions from paddy field in Taihu region. Environ. Geochem. Hlth. 26, 209–219. International Rice Research Institute, 1987. Physical measurements in flooded rice soil. The Japanese Methodologies, pp. 42–44. Ju, X.T., Xing, G.X., Chen, X.P., Zhang, S.L., Zhang, L.J., Liu, X.J., Cui, Z.L., Yin, B., Christie, P., Zhu, Z.L., Zhang, F.S., 2009. Reducing environmental risk by improving N management in intensive Chinese agricultural systems. PANS 106, 3041–3046. Kim, J.S., Oh, S.Y., Oh, K.Y., 2006. Nutrient runoff from a Korean rice paddy water shed during multiple storm events in the growing season. J. Hydrol. 327, 128–139. Li, Q.K. (Ed.), 1992. Paddy soils of China. Science Press, Beijing (in Chinese). Liang, X.Q., Xu, L., Li, H., He, M.M., Qian, Y.C., Liu, J., Nie, Z.Y., Ye, Y.S., Chen, Y.X., 2010. Influence of N fertilization rates, rainfall, and temperature on nitrate leaching from a rained winter wheat field in Taihu water shed. Phys. Chem. Earth, http://dx.doi.org/10.1016/j.pce.2010.03.017.
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